SCHEINER S EXPERIMENT. 



255 



The lens should be moved to and from the eye until the 

 edge of the pupil falls beyond its margin ; it should also be 

 slightly inclined from the perpendicular in order to prevent 

 the disturbance caused by reflections from its surface. 



The two methods should be practised upon the eye of 

 .another person. 



Accommodation. Scheiner's experiment. Prick two holes 

 in a card near each other, so that they fall within the diameter 

 of the pupil, hold the card close to the eye with the holes 

 placed horizontally and look through them simultaneously at 

 two pins stuck vertically into corks (which rest upon a table), 

 the one at 60 cm and the other at 25 cm from the eye in the 

 same line of vision. 



FIG. 53. N and F the near and far pins. - D the card with its two holes in each 

 case. R the blocked aperture. C reversal by the brain. The course of the rays of 

 li.uht from the non-focussed object are represented by dotted lines. 



First focus the distant pin, three images will appear. The 

 central one is the single sharp image of the further pin. 



Prove that the two side ones are separate images of the near 

 pin, which are insufficiently converged to fall upon the same 



